“Spider-Man: Turn Off the Dark” is not only the most expensive show in Broadway history — it may also be the most dangerous.

Yet another actor has been injured — the third in about a month and a half — and the show’s opening night isn’t even until Jan. 11.

Natalie Mendoza, who plays the villainous character Arachne, was hit on the head by a rope at the first preview Sunday night.

Mendoza, whose character bites Peter Parker and turns him into Spider-Man, felt fine and performed again Wednesday night.

But she fell ill on Thursday and could not perform that night. Her absence was first reported by BroadwayWorld.com.

Mendoza was diagnosed with a concussion, and she’ll be out of the show at least a week.

Her understudy, America Olivo, a former Playboy cover model, will take over the role.

It’s the third accident to beset the troubled, $65 million extravaganza that boasts the most complicated aerial stunts ever assembled for a Broadway show.

In October, aerialist Kevin Aubin broke both his wrists when he slammed down onto the stage during a flying stunt performed as part of a presentation for ticket brokers and group-sales agents.

In a separate incident, another actor broke his foot while rehearsing the show.

A source close to Actors’ Equity, which represents the actors and looks after their safety on Broadway, told The Post: “I’m very disappointed in Equity. They seem to have been somewhat lax about safety from the beginning.”

“Spider-Man” lead producer Michael Cohl yesterday issued a statement saying that “Natalie Mendoza suffered an injury earlier this week, and on doctors’ orders will be out of the show for a brief period. We look forward to welcoming her back to the production very soon.”